In order to facilitate plastic products (e.g. a plastic zipper puller) having better handle, a plurality of pointy projections are arranged on surfaces of the zipper puller. Therefore, a plurality of corresponding coining holes are needed to be formed in a mold cavity of the zipper puller. A prior manufacturing method includes firstly engraving holes to be coined in the mold cavity according to the pointy projections of the zipper puller, then molding the mold cavity by a coining die with hardened tips.
However in the prior method, shape of the holes to be coined which are formed by engraving and milling is greatly different from shape of the pointy projections, thus the coining area is large, and material being coined is excessively extruded. It is easy to cause material around the holes being protuberant. Thereupon, post processing for flattening is required after the coining process is finished. If a coining depth is larger, it will result in damage to side walls of the coining holes in the mold cavity.
At the same time, since both of the area and the depth of the coining process are large, not only the cracking of the coining holes during the coining process is easy to occur, but also it is difficult to make an optimal cusp on the end of the coining holes because the radius of the cusps is normally larger than 0.05 mm.